Do we have the right cell target in MS? Yes and no; we need a multicellular approach.
Recently the attention in MS has been on the B-cell as if it was the holy grail of MS treatments. It is not.
In several posts, over the last few weeks, I have made the case that the B-cell is important, probably as an antigen presenting cell, but it is not the ‘be all and end all’ of MS treatments. It is clear that rebound post-natalizumab is driven my B-cells and the positive data on the first BTK inhibitor would indicate that the B-cells are working via the B-cell receptor on antigen presentation. If only we knew what these antigens were we would have a much better handle on the cause of MS.
I know this science stuff is hard, but it is important. At the end of the day, the nut and bolts of MS must be molecular; molecules mean treatment targets and potentially more focused and hopefully better and safer treatments in the future.
I have stressed that simply targeting B-cells in both the periphery and central nervous system will not be enough to effectively treat MS in the long-term. When we look at end-organ damage markers in pwMS who are on B-cell therapies they have ongoing brain volume loss, albeit at a lower rate, and enlarging lesions (T1 black holes), which are both indicative of ongoing smouldering MS. So what do we need to do? I have provided circumstantial evidence that NIRTs (non-selective immune reconstitution therapies) have a slight edge on the B-cell therapies and this may be because they are also targeting T-cells. The latter, however, comes at a price of greater adverse events in relation to immunosuppression. The proportion of MSers on NIRTs who experience disability improvement seems higher when compared to the anti-B cell agents, which indicates that NIRTs are doing something else over and above their effect on the B-cell compartment. However, based on their overall safety profile it is unlikely that the NIRTs (alemtuzumab & HSCT) will be a therapeutic strategy that the wider MS community will adopt with vigour. Although from comments on this blog there is an informed group of MSers who feel hard done by because their HCPs won’t offer them the option of using NIRTs first-line, i.e. very early in the course of their disease when they have the most to gain from these therapies.
Is there anything else we can do to improve on the profile of B-cell therapies to make them better? Yes, I think there is. Targeting the plasma cell,in addition to the B-cell. Data on plasma cells goes back decades and surprisingly the plasma cell has never been a major therapeutic target in MS. John Prineas, one of my MS heroes, has always stressed the importance of the plasma cell in MS. His paper below from 1978 documents just how enriched the brains of MSers are with this population of cells. What is often not stressed is that the biology of plasma cells is so so different to the B-cell, which opens up new therapeutic targets that are quite different to those in the T and B cell compartments. More on this topic another time.
You are aware of the recent publication showing that about 55% of Polish MSers treated with intravenous cladribine lost their oligoclonal bands 10 or more years after treatment and if they did lose their OCBs they tended to have lower EDSS scores. We have known for years that MSers, with either relapse-onset or primary progressive diseases, who don’t have OCBs do better. There is also evidence from biomarker and pathology studies that the OCBs may be driving several of the disease processes that have been linked to advanced or progressive MS, i.e. microglial activation and grey matter pathology. Based on these observations, we hypothesise that OCBs are very likely to be pathogenic in MS, which is why we are embarking a research programme to try and target the plasma cells within the CNS of MSers. Do you think we are crazy?
To get a handle on the plasma cell we are going to have to study what happens in the spinal fluid. There are simply too many plasma cells in the periphery which will drown out any signal from the CNS. To participate in the studies we are planning we will have to perform serial, annual, lumbar punctures or spinal taps to see if our add-on therapy is killing and/or reducing the number of plasma cells in your brain and spinal cords. The good news is that we have de-risked the lumbar puncture with the use of atraumatic needles and screening. I never thought I would be saying this but most of our patients don’t mind having LPs, particularly when they understand the reason behind the LP. CSF neurofilament levels are now part of our prognostic profile of MSers at baseline and we are increasingly using them to assess response, or lack of response, to treatment. So if you want to be treated and treated-2-target beyond NEDA, and beyond the B-cell, then having an LP is important.
We hope our proposed plasma cells studies will lead to a mindset that goes beyond the B-cell to target some of the mechanisms that are responsible for smouldering MS.
Prineas & Wright. Macrophages, lymphocytes, and plasma cells in the perivascular compartment in chronic multiple sclerosis. Lab Invest. 1978 Apr;38(4):409-21.
Perivascular cells in CNS tissue from six multiple sclerosis (MS) patients and a patient with motor neuron disease were examined by light and electron microscopy. Lymph node tissue from one MS patient was also examined. CNS perivascular macrophages in both MA and motor neuron disease were found to closely resemble free macrophages elsewhere in the body except that they often contained unusually large primary lysosomes. Cytoplasmic inclusions consisting of membrane-bound stacks of curved linear profiles, presumed to be a product of myelin degradation, were constantly observed in microglia in MS plaques but were rarely observed in perivascular macrophages in the same area. Unidentified cylindrical bodies were observed within cysternae of rough endoplasmic reticulum in some lymph node cells. Quantitative studies of the perivascular cell population in one MS case revealed, in histologically normal white matter 260 lymphocytes and 178 plasma cells per cubic millimeter of fresh tissue. Typical chronic plaque tissue without obvious inflammatory cell cuffing contained 1772 plasma cells per cubic millimeter of fresh tissue. No plasma cells were observed in the CNS in motor neuron disease. The results of this study suggest that perivascular macrophages in the CNS represent a specialized population of monocyte-derived free macrophages, that these cells differ functionally from microglial cells, and that the digestion of myelin breakdown products in MS requires the participation of both cell types. The results also suggest that in some chronic MS cases there is a large, permanent population of CNS plasma cells that persists, like the elevated cerebrospinal fluid IgG level in this disease, for the life of the patient, that these cells, rather than inflammatory cells in fresh lesions, are the major source of this raised IgG, and that the existence of such a population of cells may indicate the continuing expression of antigens in chronic MS lesions in the absence of fresh lesion formation.